The present invention relates to a driving method for an active-matrix-type display using switching elements, and also concerns a liquid crystal display driven by such a driving method.
Recently, along with the development of the information-dependent society, mobile information terminals, in particular, personal data asistances (PDA), have received much attention. One of objectives with these personal data assistances is to achieve low power consumption, and liquid crystal displays have been mainly used as the display device thereof.
Liquid crystal displays are mainly classified into two types, that is, the passive-matrix type and the active matrix type, and the latter type has superior features in the display quality. The active-matrix-type displays are classified into two types, that is, those using three-terminal elements such as TFTs (Thin Film Transistors) and those using two-terminal elements such as MIM (Metal-Insulator-Metal) devices, as the switching elements thereof. The latter type is advantageous in that the manufacturing process is simpler as compared with the former, making it possible to achieve low costs, and in that the two-terminal construction makes the electrode wiring simpler and the device smaller, resulting in a higher aperture ratio in pixels.
FIG. 6 shows the construction of a conventional active-matrix-type liquid crystal display 51 using two-terminal elements. The liquid crystal display 51 is provided with a display panel section 60 having a structure in which a liquid crystal layer is sandwiched by a pair of substrates. More specifically, as indicated by an equivalent circuit in FIG. 7, the display panel section 60 has an arrangement in which two-terminal elements 72 and liquid crystal display elements 71 (display elements) are series-connected for each unit area between a plurality of scanning electrodes Yi (i=1, 2, . . . , m) and a plurality of data electrodes Xj (j=1, 2, . . . , n) that are arranged in directions so as to intersect each other, and areas corresponding the respective liquid crystal display elements 71 are arranged in a matrix format as pixels.
A driver 64 for scanning-electrode signals selects the respective scanning electrodes Yi in a line-sequential manner for each frame period and applies a predetermined selection voltage thereto, and is normally constituted by a control section, shift registers, analog switches, etc. A driver 62 for data-electrode signals applies a predetermined data signal voltage corresponding to display data to respective data electrodes Xj that are in the selection period. Thus, the selection voltage and the data signal voltage are applied to the respective pixels during the selection period. A voltage difference between the selection voltage and the data signal voltage allows a charge corresponding to the display data to be accumulated. This charge is maintained by the two-terminal elements 72 until the next selection period so that the display state is maintained during one frame period. In other words, the display state is desirably controlled on the display panel section 60 by applying predetermined voltages to the respective ends of each pixel.
In order to display external input information on the display panel 60, a control section 65 sends control signals to a voltage-forming circuit 61 for forming a voltage to be applied to the driver 62 for data-electrode signals as well as to a voltage-forming circuit 63 for forming a voltage to be applied to the driver 64 for scanning-electrode signals. Input signals to the control section 65 consist of a scanning start signal S, a scanning clock LP, a clock CLK, a display data signal DATA, a data enable signal ENAB, a power-supply signal DISP, etc. Among these, the scanning clock LP, the clock CLK, the display data signal DATA and the data enable signal ENAB are outputted to the voltage-forming circuit 61 and the driver 62 for data-electrode signals, while the scanning start signal S and the scanning clock LP are outputted to the voltage-forming circuit 63 and the driver 64 for scanning-electrode signals.
A power-supply voltage for driving the liquid crystal display 51, not shown, is sent to the voltage-forming circuits 61 and 63. Thus, the power-supply voltage and the respective signals sent from the control section 65 are used to form voltage waveforms to be applied to the data electrode Xj and the scanning electrodes Yi.
FIG. 8 shows one example of a timing chart of the above-mentioned input signal. When a selection voltage, not shown, is applied to the respective scanning electrodes Yi during a selection period corresponding to the frequency of the scanning clock LP, the display data signal DATA (a signal corresponding to the xe2x80x9cHIGHxe2x80x9d period of the data enable signal ENAB), which has a data signal voltage sent in synchronism with the clock CLK, is outputted to the respective data electrodes Xj so that a voltage corresponding to a difference between the selection voltage and the data signal voltage is applied to the respective pixels. The signal indicating the scanning start of one frame is the scanning start signal S, and the scanning clocks LP the number of which is not less than the number of scanning electrodes are present within one period of the scanning start signal S. These signals are generated and supplied while the power-supply signal DISP goes high, from the application of the power until the liquid crystal display 51 to the cut-off thereof.
The active-matrix type liquid crystal display 51 using two-terminal elements 72 has a problem in which a sticking phenomenon occurs due to variations in the voltage-current characteristics of the two-terminal elements 72. In order to solve this problem, U.S. Pat. No. 5,760,758 (published on Jun. 2, 1998) and U.S. Pat. No. 5,663,744 (published on Sep. 2, 1997) have disclosed a driving method in which, during the selection period of the respective scanning electrodes, a voltage is switched to a plurality of levels and applied to the scanning electrodes. FIGS. 9 and 10 show examples of waveforms to be applied to the display panel in the above-mentioned method.
FIG. 9 shows the construction of U.S. Pat. No. 5,760,758 in which a voltage to be applied to the scanning electrodes for one selection period is switched to two levels. Supposing that the liquid crystal display to which this driving method is applied has the same structure as the liquid crystal display 51 of FIG. 6, signals, sent from the control section 65 and the voltage-forming section 63 to the driver 64 for scanning-electrode signals, allow signals 85 and 86 having voltage waveforms as respectively shown in the Figure to the scanning electrodes Yi and Yi+1, respectively. In the same manner, the signals, sent from the control section 65 and the voltage-forming section 61 to the driver 62 for data-electrode signals, and the display data signal DATA allow a signal 87 having a voltage waveform indicated by a solid line or a broken line in accordance with the display data to be supplied to the data electrodes Xj. Signals 81 to 84 are part of signals generated in the control section 65; and signal 81 is a scanning start signal S for determining the scanning start for one frame, signal 82 is a scanning clock LP for deciding one selection period, signal 83 is a signal for controlling the pulse width with respect to the voltage levels to be applied to the scanning electrodes Yi during one selection period, and signal 84 is a signal for determining the polarity of the respective voltage levels.
FIG. 10 shows the construction of U.S. Pat. No. 5,663,744 in which a voltage to be applied to the scanning electrodes for one selection period is switched to three levels. Similarly, supposing that the same structure as the liquid crystal display 51 of FIG. 6 is used, signals, sent from the control section 65 and the voltage-forming section 63 to the driver 64 for scanning-electrode signals, allow signals 95 and 96 having voltage waveforms as respectively shown in the Figure to the scanning electrodes Yi and Yi+1, respectively. In the same manner, the signals, sent from the control section 65 and the voltage-forming section 61 to the driver 62 for data-electrode signals, and the display data signal DATA allow a signal 97 having a voltage waveform indicated by a solid line or a broken line in accordance with the display data to be supplied to the data electrodes Xj. Signals 91 to 94 are part of signals generated in the control section 65; and signal 91 is a scanning start signal S for determining the scanning start for one frame, signal 92 is a scanning clock LP for deciding one selection period, signal 93 is a signal for controlling the pulse width with respect to the respective voltage levels to be applied to the scanning electrodes Yi during one selection period, and signal 94 is a signal for determining the polarity of the respective voltage levels.
However, in the above-mentioned conventional liquid crystal display 51, upon turning the power switch off, simultaneously as the power-supply signal DISP falls as shown in FIG. 11, all the signal supplies including the selection voltage and the display data signal DATA are stopped at once due to the termination of the driver output control signal DSPOF for determining the presence or absence of the driver output and other control signals. Therefore, with respect to the pixels that have been lit up immediately before turning the power switch off, the electric charge corresponding to the lit-up display is left accumulated for a while even after turning the power switch off due to the charge maintaining effect of the two-terminal elements 72 serving as the charge-maintaining elements installed in the display panel section 60, with the result that the display pattern immediately before turning the power switch off remains, causing a problem of remaining images.
Moreover, in the method as shown in FIG. 9 or FIG. 10 in which one selection period is driven while switching the voltage to several voltage levels, supposing that a voltage at the border between the conducted state (ON state) and the non-conducted state (OFF state) of the two-terminal elements 72 is defined as a threshold voltage, all the two-terminal elements are once switched to the ON state by applying a voltage exceeding the threshold voltage at the start of one selection period so that a charge is accumulated in the liquid crystal; thereafter, in the case of the lit-up display, a voltage for maintaining the accumulated state of the charge is applied at the succeeding stage, and in the case of the non-lit-up display, such a voltage as to draw the charge is applied at the succeeding stage.
With this arrangement, the characteristics of all the two-terminal elements in the display panel section 60 are uniformly maintained so that it is possible to reduce the occurrence of the sticking phenomenon. In this driving method, however, in the case when, for example, switching is made to three voltages, upon turning the power switch off while the selection voltage is being applied to the scanning electrodes Yi, a voltage (a driver output for scanning electrode signals) VCOM, indicated by a solid line in FIG. 12 and FIG. 13, is applied to the scanning electrodes Yi, with the result that pixels on the scanning electrodes Yi are lit up, causing a residual display image. Moreover, in this case, since a dc voltage is applied to the liquid crystal display elements 71 for a long time, the liquid crystal is adversely affected and subjected to deterioration.
Moreover, in the case of the light-transmitting type liquid crystal display, since the backlight is turned off simultaneously as, or immediately before the display is turned off, this remaining image is not so conspicuous; however, in the case of the reflection type liquid crystal display, since external light is not shielded, this remaining image becomes very conspicuous.
Here, this problem of remaining images also arises in the case when three-terminal elements such as TFTs are used as switching elements.
The present invention has been devised so as to solve the above-mentioned problems, and its objective is to provide an active-matrix-type display using switching elements which causes no remaining image at the time of turning the power switch off, and a liquid crystal display using such a driving method.
In order to achieve the above-mentioned objective, the driving method for a display device of the present invention, which is applied to a display device that is provided with a plurality of scanning electrodes and a plurality of data electrodes that are arranged in directions so as to intersect each other, display elements having respective pixels which are arranged in a matrix format and have their display state determined by controlling a quantity of charge, and switching elements, installed in the respective pixels, for switching a charging current to the pixels, is arranged so that, with respect to the scanning electrodes, selection is line-sequentially made for each frame period and a selection voltage is applied thereto, and to the data electrodes is applied a data signal voltage corresponding to a display state, so that the pixels are charged. This driving method for a display device is further characterized in that, when a power-supply signal indicating that the power supply of the display device is to be turned off is detected, after all the pixels have been switched to the non-lit-up state, the application of the above-mentioned selection voltage to the scanning electrodes and the application of the above-mentioned data signal voltage to the data electrodes are stopped.
In the above-mentioned invention, at the time when, upon turning off the power supply of the display device, a power-supply signal indicating that the power supply of the display device is to be turned off is detected, corresponding charges have still been accumulated in the pixels; therefore, the accumulated charge of each pixel is discharged so as to set to the quantity of charge corresponding to the non-lit-up display so that all the pixels are switched to the non-lit-up state, that is, so that the display elements are switched to the non-lit-up state on the entire surface. Then, after this state has been achieved, the application of the above-mentioned selection voltage to the scanning electrodes and the application of the above-mentioned data signal voltage to the data electrodes are stopped.
With this arrangement, no pixels in the lit-up state exist in the display elements at the time of completion of the display; therefore, it becomes possible to prevent the occurrence of residual image resulting from a display image immediately before the turning off, even after the power supply of the display device is turned off.
In order to achieve the above-mentioned objective, the driving method for a display device of the present invention is characterized in that a non-display period in which the selection voltage is not applied to any of the scanning electrodes is formed in one frame period, and in that after all the pixels have been switched to the non-lit-up state after the detection of the power-supply signal, the application of the above-mentioned selection voltage to the scanning electrodes and the application of the above-mentioned data signal voltage to the data electrodes are preferably stopped in synchronized timing with an inter non-display period signal.
In the above-mentioned invention, the non-display period is provided, and after the power-supply signal indicating that the power supply of the display device is to be turned off has been detected, the pixels maintained in the lit-up state are switched to the non-lit-up state, and the display is then turned off in the non-display period. The timing in which the display is turned off may be any time as long as it is within the non-display period and as long as it takes place after the display state of the pixels corresponding to scanning electrodes last selected has been switched to the non-lit-up state; therefore, the timing is synchronized with the inter non-display period signal after a period required for setting the non-lit-up state has elapsed. The application of this timing allows the pixels that were lit up prior to the last lit-up-state of pixels to be inevitably switched to the non-lit-up state.
In this manner, the non-display period is provided within one frame period, and after the entire surface has been switched to the non-lit-up state, the display elements are turned off so that it becomes possible to prevent the dc voltage from being applied to the display elements.
In order to achieve the above-mentioned objective, the driving method for a display device of the present invention is characterized in that the timing in which the application of the above-mentioned selection voltage to the scanning electrodes and the application of the above-mentioned data signal voltage to the data electrodes are stopped is set to be synchronous to the scanning start signal of one frame.
In the above-mentioned invention, the timing in which the application of the above-mentioned selection voltage to the scanning electrodes and the application of the above-mentioned data signal voltage to the data electrodes are stopped is synchronized by the scanning start signal that determines the start of scanning of one frame. In other words, the scanning start signal is utilized as the inter non-display period signal that allows the timing for making the display turn off to be synchronized. The application of the scanning start signal makes it possible to prevent the dc voltage from being applied to the scanning electrodes.
Therefore, a simpler driving method can be provided by utilizing the existing signal, in an attempt to turn off the display after the display elements have been switched to the non-lit-up state over the entire surface.
In order to achieve the above-mentioned objective, the driving method for a display device of the present invention is characterized in that the timing in which the application of the above-mentioned selection voltage to the scanning electrodes and the application of the above-mentioned data signal voltage to the data electrodes are stopped is set to be synchronous to the start of one selection period for the scanning electrodes.
In the above-mentioned invention, the timing in which the application of the above-mentioned selection voltage to the scanning electrodes and the application of the above-mentioned data signal voltage to the data electrodes are stopped is synchronized by the signal such as a scanning clock that determines the start of one selection period for the scanning electrodes. In other words, the signal for determining the start of one selection period is utilized as the inter non-display period signal that allows the timing for making the display turn off to be synchronized. The application of such a signal makes it possible to prevent the dc voltage from being applied to the display elements.
Therefore, a simpler driving method can be provided by utilizing the existing signal, in an attempt to turn off the display after the display elements have been switched to the non-lit-up state over the entire surface.
Moreover, in order to achieve the above-mentioned objective, the driving method for a display device of the present invention is characterized in that, upon detection of the power-supply signal, a controlling process is started so as to switch the display elements to the non-lit-up state over the entire surface, and in that after a period of time not less than the response time of the display device has elapsed since the start of the controlling process for the non-lit-up state of the pixels corresponding to the scanning electrodes that were lastly selected prior to the above-mentioned control, the application of the above-mentioned selection voltage to the scanning electrodes and the application of the above-mentioned data signal voltage to the data electrodes are stopped.
In the above-mentioned invention, upon detection of the power-supply signal indicating that the power supply of the display device is to be turned off, the controlling process is started so as to switch the display elements to the non-lit-up state over the entire surface. The controlling period for the non-lit-up state over the entire surface is set to last up to a point of time during which the predetermined period of time not less than the response time of the display device has elapsed since the start of the controlling process for the non-lit-up state of the pixels corresponding to the scanning electrodes that were lastly selected prior to the above-mentioned control, that is, since the start of the controlling process for the non-lit-up state of the pixels that were selected last through the line sequential scanning after the start of the controlling process.
In other words, even in the case when the control for switching the pixels in the lit-up state to the non-lit-up state is started, since there is a delay in response time before the quantity of charge corresponding to the non-lit-up state has actually been achieved, the controlling process for the non-lit-up state over the entire surface is carried out on all the pixels for a period exceeding the response time so as to provide the non-lit-up display over the entire surface, and the application of the above-mentioned selection voltage to the scanning electrodes and the application of the above-mentioned data signal voltage to the data electrodes are stopped, thereby turning the display off.
With this arrangement, the pixels maintained in the lit-up display state prior to the turning off of the display elements can be switched to the non-lit-up display state in a stable manner.
In order to achieve the above-mentioned objective, the driving method for a display device of the present invention is preferably arranged so that the display elements are maintained in the non-lit-up display state over the entire surface up to completion of a period corresponding to a predetermined number of frames after the frame in which the power-supply signal has been detected.
In the above-mentioned invention, in the case when a power-supply signal instructing the turning off of the power supply of the display device has been detected in a certain frame period, the display elements are switched to the non-lit-up display over the entire surface up to completion of a period corresponding to -a predetermined number of frames starting from the next frame. In this manner, the control period including the response time having changes in display states is completed in synchronized timing with the switching of frames; thus, it becomes possible to easily control the non-lit-up display over the entire surface.
In order to achieve the above-mentioned objective, the driving method for a display device of the present invention is preferably designed so that the predetermined number of frames is set as an even number.
In the above-mentioned invention, in the case when an inverted selection voltage is applied to pixels for each frame, with an arrangement in which the predetermined number of frames is set to an even number, since the voltage values and polarities, applied to all the pixels, are cancelled in a time-averaging manner, it is possible to prevent degradation in the liquid crystal to a minimum.
In order to achieve the above-mentioned objective, the driving method for a display device of the present invention is preferably designed so that during a period corresponding to a predetermined number of frames after the detection of the power-supply signal, prior to the non-lit-up display over the entire surface of the display elements, the display elements are switched to the lit-up display over the entire surface.
In the above-mentioned invention, after the power-supply signal instructing the turning off of the power supply of the display device, the lit-up display over the entire surface is provided for a predetermined number of frames prior to the start of the control for the non-lit-up display over the entire surface. With this arrangement, after charges corresponding to the lit-up-display have been accumulated on all the pixels uniformly, the charges are reduced to the quantities of charge corresponding to the non-lit-up display; thus, since the quantity of charge of the display panel becomes uniform on all the pixels after the non-lit-up display over the entire surface has been provided, it becomes possible to positively eliminate remaining images.
Furthermore, in order to achieve the above-mentioned objective, the liquid crystal display of the present invention, which is provided with a plurality of scanning electrodes and a plurality of data electrodes that are arranged in directions so as to intersect each other, liquid crystal display elements of a reflection type having respective pixels which are arranged in a matrix format and have their display state determined by controlling a quantity of charge, and switching elements installed in the respective pixels, for switching a charging current to the pixels, is characterized in that, with respect to the scanning electrodes, selection is line-sequentially made for each frame period and a selection voltage is applied thereto, and to the data electrodes is applied a data signal voltage corresponding to a display state, so that the pixels are charged. In this liquid crystal display, when a power-supply signal indicating that the power supply of the display device is to be turned off is detected, after all the pixels have been switched to the non-lit-up state, the application of the above-mentioned selection voltage to the scanning electrodes and the application of the above-mentioned data signal voltage to the data electrodes are preferably stopped.
In the above-mentioned invention, since the driving method for a display device of the present embodiment is applied to reflection-type liquid crystal display elements, the turning off of the display is performed after the liquid crystal display elements have been switched to the non-lit-up state over the entire surface; thus, no image patterns appear even if external light is directed onto the liquid crystal display element after the power-supply of the liquid crystal display has been turned off. Therefore, a strong remaining image after the turning off of the power supply, which is inherently caused on the conventional external light utilizing display, can be eliminated.